1. Field of the Invention
This invention relates to an electron-emitting device, an electron source and an image-forming apparatus comprising such devices and, more particularly, it relates to a method of manufacturing an electron-emitting device.
2. Related Background Art
There have been known two types of electron-emitting device; the thermoelectron type and the cold cathode type. Of these, the cold cathode type include the field emission type and the metal/insulation layer/metal type and the surface conduction type.
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. A surface conduction electron-emitting device is typically prepared by arranging a pair of device electrodes on an insulating substrate and an electroconductive film, which may be a metal oxide film, between the electrodes to electrically connecting them and subjecting the thin film to an electrically energizing process referred to as "electric forming" to locally deform or modify the thin film and produce therein an electron-emitting region.
A surface conduction electron-emitting device is a device that shows a sudden and sharp increase in the emission current Ie when the voltage applied thereto exceeds a certain level (a threshold voltage), whereas the emission current is practically undetectable when the applied voltage is found lower than the threshold. Because of this remarkable feature, the emission current of the device can be controlled through the device voltage while the emission charge can be controlled through the duration of time of applying the device voltage. A variety of image-forming apparatuses can be produced, using in combination an electron source realized by arranging a plurality of surface conduction electron-emitting devices and a phosphorous body designed to emit visible light when irradiated with electrons coming from the electron source. With this technique, emissive type display apparatuses having a large display screen capable of displaying high quality images can be produced without difficulty. Hence, such apparatuses are expected to replace CRTs in the future.
Materials that can be used for the electroconductive film of a surface conduction electron-emitting device include, besides metal oxides, metal and carbon. When a metal oxide is used, an organic metal compound is applied to the substrate to form an initial thin film of the compound and then baked in the atmosphere to produce a thin metal oxide film. Massive efforts are currently being paid to fully exploit the potential of this method because it involves a relatively simple manufacturing process and is advantageous relative to other techniques for the formation of thin films.
For the purpose of the present application, "a thin metal oxide film" can partly contain one or more than one metals in addition to a metal oxide.
A patterning operation needs to be carried out to produce an electroconductive film having a desired profile. With a conventional patterning technique, a mask having a desired pattern is formed on an initial thin film and then it is etched to remove unnecessary portions thereof. FIGS. 21A through 21F of the accompanying drawings schematically illustrates steps to be followed for a conventional patterning operation.
Step a: Electrodes 4 and 5 are formed on a substrate 1 (FIG. 21A).
Step b: An initial thin film 201 is formed on the entire surface of the substrate 1 for an electroconductive film (FIG. 21B). Typically, it is a metal film formed by vacuum deposition or sputtering.
Step c: A photoresist 202 is applied to form a layer on the entire surface of the initial thin film (FIG. 21C).
Step d: The applied photoresist is exposed to light, using a mask having a desired pattern, and photographically developed to produce a resist pattern 203 (FIG. 21D).
Step e: The portions of the initial thin film not covered by the resist pattern are removed by wet etching (FIG. 21E). Etchants that can be used for the purpose of the present invention include nitric acid. It is important to select an etchant that is noncorrosive relative to the device electrodes.
Step f: The resist pattern is removed to produce an electroconductive film 204 (FIG. 21F).
While the above technique is popularly used, it may not be used in certain cases as will be described hereinafter. If such is the case, "a lift-off technique" may be a possible alternative. A lift-off technique that can be appropriately used to produce a surface conduction electron-emitting device will be described below by referring to FIGS. 20A through 20K.
Step a: Electrodes 4 and 5 are formed on a substrate 1 (FIG. 20A).
Step b: A metal film, typically a Cr film, is formed (FIG. 20B).
Step c: A resist is applied to form a layer on the entire surface of the metal film (FIG. 20C).
Step d: The applied resist is exposed to light, using a photo-mask having a desired pattern (FIG. 20D).
Step e: The resist is photographically developed (FIG. 20E).
Step f: The Cr film of the portions not covered by the resist are etched by means of an etchant (FIG. 20F).
Step g: The remaining resist is removed to produce a complete Cr mask (FIG. 20G).
Step h: An organic metal compound solution is applied to the product of Step g to form an organic metal thin film 6 (FIG. 20H).
Step i: The organic metal compound thin film 6 is partly turned to a metal oxide thin film as it is baked (FIG. 20I). As described earlier, a metal oxide thin film may contain as part thereof one or more than one metals beside the metal oxide. The baking conditions may appropriately be selected depending on the organic metal compound used for the metal oxide thin film. If it is a complex of palladium acetate and an amine, it is typically baked in the atmosphere at 300.degree. C. for about a little more than 10 minutes.
Step j: An electroconductive thin film 3 of the metal oxide having a desired profile is formed by lifting-off the remaining Cr and removing the unnecessary portions of the metal oxide thin film (FIG. 20J).
Step k: An electron-emitting region 2 is formed in the electroconductive thin film 3 by means of an electric forming process as described earlier (FIG. 20K).
However, the above described known method is accompanied by problems, which will be described below.
In the operation of patterning by etching, the organic metal compound of the initially formed thin film needs to be pyrolyzed under appropriate conditions to produce a metal thin film, onto which resist is applied for the subsequent steps. However, the produced metal thin film is poorly adherent to the substrate and electrodes and can easily come off to totally prevent the operation from proceeding to the next step.
A conceivable method to avoid the problem of poor adhesion is to produce a metal oxide thin film in stead of a metal thin film by heat treatment at appropriate temperature in an oxidizing atmosphere. However, a metal oxide thin film is less liable to be etched with an ordinary etchant such as nitric acid and, therefore, a lift-off technique as cited above has to be normally used. A metal film such as a Cr film is used for the mask of the lift-off operation because photoresist cannot withstand the high temperature of the heat treatment of organic metal compound thin film.
Since this method involves a large number of steps, the overall yield of manufacturing electron- emitting devices of the type under consideration can become rather low. If an electron source comprising a large number of electron-emitting devices is used for an image-forming apparatus, all the devices have to operate because only a small number of defective devices, if exist, can significantly degrade the quality of images formed on the display screen of the apparatus. Thus, a low yield is a vital disadvantage in the manufacture of electron-emitting devices. An effective way to improve the yield will be to reduce the number of steps.
Additionally, the operation of forming a metal film such as a Cr film requires the use of a vacuum system such as a vacuum deposition assembly or a sputtering assembly, which is very costly, and a very large electron source comprising a number of electron-emitting devices arranged in array cannot feasibly be manufactured. This latter problem makes it abortive to fully exploit the advantage of the technique of applying an organic metal compound to produce a large processed surface area for a multiple type electron source. If, on the other hand, a lift-off technique is used to produce a large processed area, there can arise problems is in the course of processing such as exfoliation and undesired re-adhesion of thin film.
In view of the above problems and other problems, it is desired to develop a process of manufacturing electron-emitting devices that involves only a reduced number of steps and does not require the use of a vacuum system.